In an automotive environment, electromagnetic interference (EMI) is often present in the form of stray radio frequency noise, cross-talk between electrical devices, and noise created by such things as the making and breaking of circuits, spark discharges, poor or intermittent metallic contact between metal bonds and components, and atmospheric interference. It is well known that such EMI sources pose a serious threat to the electrical integrity of electrical circuitry and the function of electrical components. As the dependence on electrical circuitry by modern automobiles increases, there is an increased need for effective electrical filters to reduce electromagnetic interference between individual electrical components and circuits. The difficulty of reducing such extraneous noise is further complicated by the desire to produce automobile electronics in smaller modules. In addition, low level signals associated with on-board sensors and computer systems requires better EMI filtering as switching electronics operate at higher voltages.
Currently, the predominant method of EMI filtering is to install capacitors on an electronic circuit board using conventional manufacturing technology. At times, an inductor is added to provide "LC-type" filtering, such as when a block inductor is placed in series with one or more discrete capacitors. The use of LC-type filtering is often necessary in that a capacitor will exhibit inductance at high frequencies, producing resonance which can seriously impair the effectiveness of an electronic device. However, as electronic devices become more compact, these types of filters take up increasingly valuable space on the circuit board. Furthermore, these filters do not always provide a sufficient level of protection in that they are extremely sensitive to frequency and thus application dependent. As a result, it is often necessary to narrowly tailor the capabilities of such filters to perform well for very specific applications.
It is also known to locate EMI filters, such as feed-through filters, at electrical interconnects to suppress cross-talk and other extraneous noise at the connector pins. Simple forms of such filters include a dielectric, and more preferably a ferroelectric ceramic tube plated on its interior and exterior surfaces with a metallic coating that serves as a pair of electrodes. The interior electrode is in electrical contact with a connector pin while the exterior electrode is in electrical contact with ground. The capacitance of the filter depends upon the surface area and thickness of the tube and the dielectric constant, or permittivity, of the ceramic material used. While such filters are adequate for many applications, they are prone to exhibit the aforementioned resonance at very high frequencies.
It is known to form the ceramic tube from a ferromagnetic material such as ferrite, and then sinter a ferroelectric material, such as barium titanate, to the exterior surface of the tube. The ferromagnetic material, characterized by having high permeability, provides inductance while the ferroelectric material,characterized by having high permittivity, provides capacitance between the ferromagnetic material and ground. As a result, the ferromagnetic and ferroelectric materials act together to provide an LC-type filter, wherein the inductive capability provided by the ferromagnetic material attenuates the resonance which otherwise occurs with the capacitive element at the higher frequencies. Examples of these types of EMI filters include U.S. Pat. No. 3,035,237, to Schlicke, U.S. Pat. No. 3,243,738, to Schlicke et al., U.S. Pat. No. 3,789,263 to Fritz et al., and U.S. Pat. No. Re. 29,258 to Fritz.
While the above EMI filters have advantageous features in terms of electromagnetic interference attenuation, they are not altogether economical to manufacture for purposes of the quantities typically required in automotive applications. Furthermore,single versus multi-component connectors are simpler to assemble and are believed to be less expensive to manufacture and store.
Materials are known which exhibit both ferroelectric and ferromagnetic, or magnetoelectric, properties. One class of such materials consist of compounds having a single crystalline phase. However, the permeability and permittivity of this group of materials are generally inadequate for technical applications because the optimum magnetoelectric properties of these compounds exist only at temperatures well below room temperature.
A more recently discovered group of magnetoelectric materials are formed from composites of fine grain powders of ferrite and lead zirconate titanate (PZT) which have been sintered together for evaluating magneto-strictive and electro-strictive effects--i.e., the contraction or expansion of a material when subjected to a magnetic or electrical field. However, lead is reactive with the ferrite, yielding a composite having greatly diminished permeability and permittivity as compared to its individual constituent materials. Such losses in constituent properties are well known to those skilled in the art.
FIG. 15 is a schematic of the electrical components of a prior art filtered-header-connector including as the filter element a block inductor and discrete compacitors.
FIG. 16 is an illustration of such a prior art filtered-header-connector 180 which includes a plurality of connecting pins 182 inserted through a block inductor 184. Each connector pin includes an individual discrete compacitor 186 which is soldered to a pin and a ground 187. Such devices involve numerous manufacturing steps to assemble and are time consuming and labor intensive.
FIG. 17 is a plot of the attenuation of a filter illustrated in FIG. 16. As can be seen from the plot, the attenuation can be characterized as a band pass filter because of L-C resonance of the capacitor/conductor and limitations of the ferrite.
In the making of filtered-headed-connectors for electronic components such as those used in automotive applications or other electronic applications, a wish list of desirable properties and characteristics of such filters can be imagined. A high resistivity would prevent shorting between adjacent connector pins. A high dielectric constant material would provide improved capacitance. A high permeability material would produce inductive capabilities and, of course, mechanical strength would provide for durability. The high resistivity, dielectric constant, permeability, and mechanical strength suggests high sintering temperatures. No single material is known to provide all of these properties. A few properties may be provided by one material and the balance provided by another material. However, simply mixing two materials together will not produce a composite which achieves the desired properties because of high porosity. If the mixture is sintered to remove the porosity, the permittivity permeability can be relative low. This is because when the two materials are sintered at high temperatures to achieve the desired characteristics highlighted above, the materials chemically react with each other resulting in lower permittivity, permeability and resistivity.